The Collaborative International Dictionary of English v.0.48:

handler \handler\ n.
   1. one who trains or exhibits animals.

   Syn: animal trainer.
        [WordNet 1.5]

   2. someone in charge of training an athlete (especially a
      boxer) or a team.

   Note: The term is used sometimes sarcastically of political
         consultants: "the president's handlers".

   Syn: coach, manager.
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

handler
    n 1: one who trains or exhibits animals [syn: animal trainer,
         handler]
    2: an agent who handles something or someone; "the senator's
       campaign handlers"
    3: (sports) someone in charge of training an athlete or a team
       [syn: coach, manager, handler]

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "handler":
   agent, athletic coach, coach, conductor, driller, drillmaster,
   driver, engineer, functionary, groomer, manipulator, operant,
   operative, operator, pilot, runner, steersman, trainer